Underwater Transmission of Mesoscopic Twin-Beam States for Applications in Quantum Communication

Quantum resources can improve the security of information transmission between two parties. So far, Quantum Communication protocols have been implemented at the single-photon level by means of entangled states [1]. In contrast to this domain, where the presence of losses can reduce the transmission rate of information [2], in the mesoscopic one the optical pulses contain sizeable numbers of photons, thus resulting more robust against any kind of external degradation [3]. In a recent work of ours, we have demonstrated that the transmission of one arm of a twin-beam (TWB) state through a lossy and noisy channel does not prevent the observation of nonclassical correlations between the two parties [4]. Based on these successful results, here we consider a more realistic scenario, in which a portion of TWB produced in a $\beta$ -Barium Borate (BBO) crystal is sent through water-filled tubes, while the other one undergoes free-space propagation [5]. We investigate the role played by the length of the tubes, the number of optical elements (OE), and the divergence of the beams through the different media. We demonstrate that, by properly acting on the light beams, we can still observe nonclassical correlations at moderate distances by using the noise reduction factor, $R$ , as a criterion for nonclassicality. The experimental implementations involve commercial photon-number-resolving (PNR) detectors. In particular, hybrid photodetectors and Silicon Photomultipliers have been used and compared.